Now I'm more confused! Why would you disable the AGP when the video card being used is AGP-based? Even so, why should my old RIVA TNT2 card work but not the FX5200?
Not all AGP chipsets or drivers are created equal. You may experience instability under linux with certain AGP chipsets, especially at higher data transfer rates. A good way to determine if AGP instability is the source of your problem is to disable the AGP driver. This means that your AGP card will basically look like a PCI card to the driver.
If you've determined that your machine is more stable with AGP disabled, then you might want to try switching AGP drivers (ie: from AGPGART to NVAGP or vice versa).